This invention relates to a solenoid-operated, pilot-controlled control valve.
Poppet-type valves have traditionally been used as on/off valves or for pressure control, while spool valves have been traditionally used for precision control. However, since poppet-type valves are less prone to contamination, there is a desire to use poppet-type valves for precision control as well.
An example of a poppet-type control valve is found in U.S. Pat. No. 3,893,471 issued to Byers, Jr. in 1975. Such valves have a pressure-balancing pin which is used to hydrodynamically balance the pilot poppet so that the only forces which act on the pilot poppet are a spring force and a solenoid coil force, thereby making the valve insensitive to load variations. However, in such a valve, when its solenoid is off, there is a fluid leakage path from inlet to outlet between the pressure balancing pin and the armature. Such leakage is acceptable when the solenoid is "on" and the valve is open. However, when the solenoid is off, such leakage can lead to "drift" of the actuator or device controlled by the valve.
One solution to this problem is described in U.S. Pat. No. 4,494,726 issued Jan. 22, 1985 to Kumar et al and assigned to the assignee of this application. The '726 valve includes a check valve in the leakage path downstream of the pressure-balancing pin. However, the space taken up by this check valve means that the initial movement of the armature will only open the check valve, thereby leaving less armature movement available for control purposes. This check valve arrangement also increases the complexity and cost of the valve while adversely effecting reliability.
An alternative to the check valve solution of '726 would be to provide an O-ring seal between the pressure-balancing pin and the wall of the armature bore which receives it. However, such a sealing arrangement would produce a friction or sticking problem which would adversely effect smooth valve operation.
Furthermore, both the '471 and '726 valves have armatures which must be inserted into the valve assembly after insertion of the pressure-balancing pin. Such an assembly is difficult to align because of the close fit between the armature and pressure-balancing pin it receives. Also, both of these valves have inlet pilot passages or orifices which may be blocked by particles of debris in the hydraulic fluid.